All cells must sense changes in their environment and respond appropriately. In this regard, the two-component signal transduction regulatory system was initially described in prokaryotic organisms where it is thought to play a function in chemotaxis, osmoregulation, sporulation, host-pathogen interactions and response to carbon, nitrogen and phosphate availability. In these microorganisms, the prototypical two-component regulator system is comprised of two proteins, a histidine protein kinase (also called a sensor protein and usually cell membrane-bound) and a response regulator (or effector protein), which is associated with an internal response. The sensor kinase, when activated by a signal, autophosphorylates a histidine residue using ATP as a phosphodonor; the histidine is a part of a conserved block of residues, typically referred to as the H-box. Subsequently, the phosphorylated sensor kinase serves as a phosphodonor to a conserved aspartate residue in the response regulator. This phosphorylation modulates the activity of the effector protein to elicit an adaptive response to the stimulus (reviewed in Hoch and Silhavy, Two-component signal transduction, ASM Press. Washington, D.C. USA (1995)).
Although the general sequence of events and the number of proteins involved is similar for all of these organisms, each pathway exhibits some variation on the basic scheme (Appleby et al., Signal transduction via the multi-step phosphorelay; not necessarily a road less traveled, Cell 86, 845-848 (1996)). For instance, in Bordetella pertussis, the BvgS-BvgA two-component modulates the transcriptional control of several virulence factors. Although there are two proteins, four phosphorylation events occur in sequence, creating a four-step His-Asp-His-Asp phosphorelay (Uhl and Miller; Integration of multiple domains in a two-component sensor protein: the Bordetella pertussis BvgAS phosphorelay, EMBO J. 15, 1028-1036 (1996)). A similar mechanism has been the plant pathogenic bacterium, Pseudomonas syringae.
Homologous pathways have recently been identified in several eukaryotic organisms, including, Saccharomyces cerevisiae, Dictyostelium discoideum, Neurospora crassa and Arabidopsis thaliana. In S. cerevisiae the phosphorelay through a two-component signal pathway is composed of three proteins. An Sln1p transmembrane protein serves as a sensor protein, which after autophosphorylation of a histidine residue and transfer to an aspartate in the same protein, phosphorylates a histidine residue of a second protein (Ypd1p). Ypd1p is a small cytoplasmic protein, which functions much like a sensor protein and, in turn, it phosphorylates a third protein effector in the relay system (Ssk1p). The activation of a downstream MAP kinase cascade is dependent upon the phosphorylation of Ssk1p. In cells which are grown under low osmotic conditions, phosphorylated Ssk1p does not activate the Map kinase pathway. However, under conditions of hyperosmolarity, phosphotransfer among the two-component does not occur. Consequently, the MAP kinase pathway and the transcription of genes involved in glycerol metabolism occur. This pathway, referred to as the HOG pathway (High Osmolarity Glycerol Response), thus provides a phosphorylated effector molecule which is inactive in environmentally stressed conditions. In D. discoideum two different histidine kinases (DhkA and DokA) have been described. DhkA modulates the transcriptional regulation of prestalk gene expression and the control of the terminal differentiation pathway. DokA is involved, like Sln1p in S. cerevisiae, in the osmoregulatory pathway. In N. crassa, a two-component histidine kinase (Nik-1) has been reported to be involved in hyphal development and osmosensitivity. Finally, in A. thaliana the product of the ETR1 gene may be involved in an early step in ethylene signal transduction through phosphorylation, as in the prokaryotic two-component systems. Thus, there is a need for the discovery of proteins responsible for causing diseases resulting from infection with pathogenic fungi because such proteins may be used in the development of treatments for such diseases.